<!DOCTYPE html>
<style>
#div {
  box-shadow: -50px 0 green;
  position: absolute;
  top: 100px;
  left: 100px;
  width: 100px;
  height: 100px;
  background-color: blue;
}
</style>
Tests paint invalidation of a verticle-rl div with overflow. Passes if no under-invalidation reported and shows no red.
<div id="div"></div>
